An obturator seals high pressure gas between a projectile and a wall of a launch tube. Providing a good gas seal increases the gas pressure in the launch tube, which increases the projectile's velocity at launch.
The obturator preferably falls off the projectile after the obturator exits the launch tube. Sometimes the obturator fails to release from the projectile, thereby increasing drag and weight of the projectile, and possibly unbalancing the projectile, thereby reducing its range and accuracy.